In recent years, a sound signal output device in a system such as a CD player, a DVD player, a radio, a TV or the like is widely used to output a sound signal to an amplifier. Then, the amplifier executes amplification process and the like for the sound signal before outputting the sound signal to a speaker or the like.
The sound signal output device outputs various control commands to the amplifier. For example, a navigation system with an audio-visual function transfers control commands such as a sound quality setting command, a mute command, a mute release command, and the like to the amplifier in a vehicle. Then, the amplifier, in response to these commands, executes process operations such as sound quality setting changes, muting of sounds, release of muting and the like upon receiving those commands. Further, the sound signal output device releases hardware mute for itself immediately before outputting the sound signal to the amplifier in some cases. For example, the navigation system keeps the hardware mute until sound source is switched from one source to the other so that a switching noise will not be transferred to the amplifier in an occasion that the navigation system begins to receive a radio program.
However, when a system having the sound signal output device in connection with the amplifier by wiring in a manner described above has a trouble that prevents sound output from the speaker, all components in the system has to be examined separately in order to tracked down a cause of the trouble.